The world's principal industrial processes for fixing atmospheric nitrogen are the Haber-Bosch process and similar methodologies which combine molecular nitrogen with molecular hydrogen over solid catalysts at high temperatures and pressures. These processes require relatively large amounts of energy, are technically sophisticated, and are primarily based on the use of fossil fuels (for instance, natural gas) as the source of hydrogen. By their very nature and chemical requirements, such methodologies are appropriate only for economies of large scale which can provide the ingredients in volume, the central production facilities, and the requisite distribution systems for effective use of the process.
Alternatives to large scale industrial methods for production of ammonia have been sought with the result that considerable chemical research has been directed towards finding economically viable and less energy-consuming methods. One approach has been the use of metal oxide catalysts and the use of gaseous nitrogen from the air. Exemplifying this approach is U.S. Pat. No. 2,500,008 which describes the synthesis of ammonia from hydrogen and nitrogen which are combined with a finely divided iron oxide catalyst and subjected subsequently to ultrasonic vibrations. Another approach uses catalytic processes which synthesize ammonia from nitrogen and water without the use of elemental hydrogen using various wavelengths of photoenergy. Consistent with these developments is the use of solar energy in various forms as the sole energy source and the use of water almost exclusively as the reducing agent. Exemplifying the latter approaches and developments are the following: "Photolysis of Water and Photoreduction of Nitrogen on Titanium Dioxide", Journal of the American Chemical Society, 99:7189-7193 (1977) which describes the photoreduction of nitrogen to ammonia using titanium dioxide alone or when doped with iron, cobalt, molybdenum or nickel, or iron oxide alone; U.S. Pat. No. 4,113,590 which describes the synthesis of ammonia and hydrazine by reduction of gaseous nitrogen with water using metal oxide catalysts under the influence of ultraviolet light; and U.S. Pat. No. 4,427,510 which describes the synthesis of nitrogen-containing compounds by combining metal oxide compounds with gaseous nitrogen, a reducing agent such as water, and a source of light whose wavelengths are in the visible range provided by sunlight or artificial light.
A net result of the advances described within these publications (and the other references cited therein) has been the recognition and acceptance of several points as basic teachings in this art. These are: first, in any photocatalytic process using a metal oxide catalyst for the reduction of molecular nitrogen, no measurable reduction of nitrogen occurs without some incident light energy being added to the reaction mixture. Second, water has been the almost exclusive and unquestionably preferred reducing agent; although other reducing agents have been incidentally mentioned in the literature, there is no indication of any kind that these other agents are in any way equal to, much less superior to, water in photocatalytic processes. Third, the yields of ammonia (and other nitrogen-containing compounds) using photocatalytic processes now known in the art are notably small. Given these generally applicable axioms of this art, it will be apparent to one ordinarily skilled in this art that a process for reducing gaseous nitrogen to ammonia using a solid metal oxide catalyst which does not require the use of photoenergy and which provides substantial increases in ammonia yield when using photoenergy, constitutes a major improvement and advancement in this field.